fix(script): request animation ticks if Animations::pending_events is not empty

Fixes the test case `/_mozilla/css/css-transition-cancel-event
.html`, which was failing under a specific circumstance.

The observed sequence of events during the failing test run looks like
this:

1. Transitions start in `div1` and `div2`.

2. `div1` generates a `transitionend` event.

3. The `transitionend` event handler removes `div2` from DOM, cancelling
   its ongoing transition.

4. `div2` is supposed to generate a `transitioncancel` event in a timely
   manner, which it does not. The test fails as a result.

What is going on here? Here's a possible explaination:

1. During one invocation of `ScriptThread::handle_msgs`...

2. In step 2, `ScriptThread::update_animations_send_events` -> `Document
   ::update_for_new_timeline_value` detects the completion of the
   transition, and in response, pends the `transitionend` event.

3. In step 3, `ScriptThread::update_animations_send_events` ->
   `Animations::send_pending_events` calls the `transitionend` handler.

4. The `transitionend` event handler removes `div2`, thereby cancelling
   its ongoing transition and triggering a reflow.

5. Reflow takes place. During this, `Animations::do_post_reflow_update`
   -> `Animations::handle_canceled_animations` pends the
   `transitioncancel` event (precursor to step 4).

6. Having discovering that there was no running animation, `Animations::
   do_post_reflow_update` calls `self.update_running_animation_presence
   (_, false)`, which sends `AnimationState::NoAnimationsPresent`.

7. The invocation of `ScriptThread::handle_msgs` ends, and another
   starts. It blocks waiting for events.

8. Meanwhile, the compositor receives `AnimationState::
   NoAnimationsPresent` and stops further generation of animation ticks.

9. With no events to wake it up, the script thread is stuck waiting
   despite having the pending `transitioncancel` event (step 4).

The HTML specification [says][1] that "an event loop must continually
run [...] as long as it exists" and does not say it can block if there
is nothing to do. Blocking is merely optimization in a user agent
implementation. Pending animation-related events must be processed every
time a "rendering opportunity" arises unless the user agent has a reason
to believe that it "would have no visible effect".

Skipping the processing of animation-related events would have visible
effect if such events are indeed present. The correct implementation in
Servo, therefore, would be to request more animation ticks so that such
events are processed in a subsequent tick.

[1]: https://html.spec.whatwg.org/multipage/#event-loop-processing-model

components/script/script_thread.rs

@@ -1709,7 +1709,7 @@ impl ScriptThread {
         }
 
         for (_, document) in self.documents.borrow().iter() {
-            document.animations().send_pending_events();
+            document.animations().send_pending_events(document.window());
         }
     }